1. Field of the Invention
The present invention relates to a method of fabricating optical devices generally referred to as photoconductor coupled liquid crystal light valves of photoaddress type liquid crystal light valves (hereinafter, referred to as photoconductor coupled liquid crystal light valves) which utilize the photoconductive effect of photoconductive layers and the electro-optical effect of liquid crystals. These photoconductor coupled liquid crystal light valves are used in image display apparatus, image processing apparatus, optical information processing systems, or the like.
2. Description of the Related Art
Recently the photoconductor coupled liquid crystal light valves utilizing the photoconductive effect of photoconductive layers and the electro-optical effect of liquid crystals have been drawing attention as optical devices to be used in image display apparatus, image processing apparatus, optical information processing systems, and the like. FIG. 6 illustrates the basic construction of a photoconductor coupled liquid crystal light valve, which as shown in the figure comprises a photoconductive layer 1, a reflection layer 5, a liquid crystal layer 2, orientation processing films 6a and 6b, two transparent electrodes 3a and 3b sandwiching them and disposed on glass substrates 7a and 7b, respectively, and an external power supply 4 which serve as a voltage applying means for applying a voltage to the transparent electrodes. Means for writing into and image-reading from the device, although not shown, are of course included therein.
The basic operating principle of this device is described below, where for simplicity the impedances of the reflection layer 5 and the orientation processing films 6a and 6b are assumed to be far lower than those of the photoconductive layer 1 and the liquid crystal layer 2.
First, an a.c. voltage V.sub.0 is applied to the photoconductive layer 1 and the liquid crystal layer 2 via the transparent electrodes 3a and 3b from the external power supply 4. The voltages applied to the liquid crystal layer and the photoconductive layer in this case are at values resulting from allocating the voltage V.sub.0 in proportion to the impedances of the photoconductive layer 1 and the liquid crystal layer 2. The voltage which will be applied to the liquid crystal layer 2 without incident light thereon is so set as to be adequately smaller than the threshold voltage (V.sub.s1) at which the liquid crystal layer will yield the electro-optical effect. This means that no electro-optical effect is not produced by the liquid crystal layer 2 at the initial state.
Assume that the region to which the light is applied is PC.sub.1 and that to which the light is not applied is PC.sub.d. When light is applied to the photoconductive layer 1, the impedance (Zp) of the region PC.sub.1 of the photoconductive layer 1 greatly decreases to a value far smaller than the impedance of the liquid crystal layer 2, which implies that most of the voltage V.sub.0 greater than V.sub.s1 is applied to the liquid crystal layer 2. As a result, there develops an electro-optical effect to the liquid crystal layer 2 of the region PC.sub.1. In contrast to this, the liquid crystal layer 2 of the region PC.sub.d remains having the initial voltage applied thereto, without causing the electro-optical effect. Thus, the light pattern (optical information) written into the photoconductive layer 1 has been formed on the liquid crystal layer 2.
The light pattern may be reproduced by applying reproductive light to the liquid crystal side from the light source (not shown).
According to the above-described operating principle, when the reproductive light is applied, the light that has penetrated the liquid crystal layer 2 and reached the reflection layer 5 will permeate up to the photoconductive layer 1 without being fully reflected by the reflection layer 5. This permeated light may cause the impedance of the photoconductive layer of the relevant region so as to disturb the image pattern of written light, disadvantageously.
In fact, optical reflection layers for ordinary use employ a layer provided by stacking thin layers in two types having a great difference in the induction rate to about 10 to 20 layers with a thickness of 1/4 of the wavelength of the readout light. However, nonuniformities in the precision of film thickness as well as in surface orientation make it greatly difficult to attain a reflectance of 99.9% or more. For this reason, there has been a problem that the reading light may permeate into the photoconductive layer to disturb the image pattern. This problem has been remarkable especially when the amount of reading light is very large, such as when a liquid crystal light valve is used as an image display device.
As a means for solving the problem, it has been proposed that an optical shielding layer which absorbs the reproductive light is provided between the reflection layer 5 and the photoconductive layer 1 (see FIG. 6). For such an optical shielding layer previous proposals have been such that a layer in which a pigment has been mixed into an organic material is put into use as the optical shielding layer, or that as described in Japanese Patent Laid-Open Publication HEI 3-18829 between the liquid crystal layer and the photoconductive layer there is provided a metallic optical shielding layer having an island-like shape and a size of 18 .mu.m or so for each side.
On the other hand, referring to FIGS. 5 and 6, the photoconductive layer 1 for this photoconductor coupled liquid crystal light valve has been proposed in various materials from organic to inorganic materials. For example, in Japanese Patent Laid-Open Publications SHO 57-150821, 58-34435, 58-199327, 59-81627, 63-253924, and the like it has been proposed that amorphous silicon be used as the photoconductive layer. The primary reason these proposals employ amorphous silicon is that it has a high sensitivity and a high resistance.
Moreover, in any of the proposals, this amorphous silicon is formed into films by a known fabrication method, the Plasma Chemical Vapor Deposition (hereinafter, referred to as P-CVD) or the sputtering method.
As will be understood from the operating principle of the device stated before, the impedance of the photoconductive layer without writing light incident thereon needs to be made as large as possible. This is because the impedance is required to be at least greater than that of the liquid crystal layer. The film thickness of such a photoconductive layer of the photoconductor coupled liquid crystal light valve is, for example according to the Japanese Patent Laid-Open Publication SHO 57-150821 and the like, 3 .mu.m or more for the thickness of the amorphous silicon layer.
However, the film formation rate by the conventional P-CVD or sputtering method is no more than 10 A/sec. This results in a prolonged depositing time for a 3 .mu.m film as long as 50 min., making the device fabrication costly.
Yet further, attaining the value of 10 A/sec. by these methods would require the RF power or gas pressure to be increased. This would accelerate reactions among plasma active seeds in vapor phase, causing (SiH.sub.2).sub.n powder to be generated during the excitation of the plasma. The powder would adhere onto the substrate under film formation, serving as the core for abnormal growth of amorphous silicon, which would lead to some image deficiency in the end device and therefore to less yield in the device fabrication.
As another problem, after film formation with the generation of powder involved as shown above, a large amount of the powder will remain within the vacuum layer of the plasma equipment. On this account, the inside of the vacuum layer needs to be cleaned before proceeding to the next fabrication process, requiring great amounts of labor and time.
In consequence, the fabrication methods as conventionally proposed would result in less-yield, higher-cost devices, which are far from feasible to put into practical use in terms of bulk production.
Moreover, in an attempt to take measures for providing the photoconductor coupled liquid crystal light valve employing amorphous silicon as its photoconductive layer as shown above with the aforementioned optical shielding layer that absorbs reproductive light, it is necessary to take out amorphous silicon from the vacuum equipment after its film formation and then move it to application equipment dedicated to organic material layer or separate film forming equipment dedicated to metal layer. Thus new equipment is necessitated for forming the optical shielding layer, further increasing the device fabrication cost. Also, foreign matters would adhere onto amorphous silicon when it is taken out, which may cause deficiencies in the optical shielding layer, optical reflection layer, and liquid crystal layer to be subsequently fabricated. These deficiencies would be causes for electrical short-circuit and the like which occur between the two transparent electrodes, resulting in deficiencies in liquid crystal image patterns.
When an organic material is employed as the optical shielding layer, there may arise a mechanical nonconformity due to a difference in thermal expansion coefficient from the amorphous silicon layer, which is of inorganic material. As a result, there have been some cases where peeling would occur at the interface between the amorphous silicon layer and the optical shielding layer. As a still another disadvantage, when a metal layer of island shape was employed, the size of one pixel would depend on the size of the island, thus restricting the enhancement of the resolution.
The Japanese Patent Laid-Open Publication SHO 59-81627 has disclosed one in which the optical shielding layer is given by amorphous silicon containing fluorine and hydrogen. This instance would involve use of SiF.sub.4 more expensive than SiH.sub.4 as the material gas, fabrication by the glow discharge method, and adjustment of the substrate temperature in preparing the optical shielding layer, thus accompanied by a difficulty in reducing the fabrication cost.
The Japanese Patent Laid-Open Publication SHO 63-253924 provides a fabrication by the P-CVD method, incapable of solving the above-described problems.
Furthermore, the inventors of the present invention have reported a method for forming a photoconductive layer made of amorphous silicon by the ECR method (The Japanese Patent Laid-Open Publication Hei 3-126920). Even in the light valve thereof, however, it was difficult to solve the aforementioned problem that the reading light may permeate into the photoconductive layer to disturb image patterns.